This invention relates generally to an apparatus and method for producing a series of dimensionally identical cast concrete structural modules which are used in the construction of multiple unit structures to be used as dwellings, storage, shelters, revetments, institutional buildings and the like. Pre-cast concrete housing modules of a generally hollow cubical or rectangular shape are being increasingly used in the construction industry, due to the fact that the units can be prefabricated away from the point of construction and, if desired, pre-wired, plumbed or otherwise modified so that the building contractor can quickly assemble the units in the field to provide the desired structure. The advantages of pre-cast concrete modules have been known in the art and are discussed and shown for example, in U.S. Pat. Nos. 3,750,366 to Rich and 3,462,908 to Wysocki.
Since the introduction of such pre-cast modules to the building industry, their use has increased, particularly as on site construction costs have steadily increased throughout the years. However, certain problems encountered with the widespread use of the modules have developed and heretofore have acted as a deterent to their further use. One of the major problems is the fact that transportation costs of a pre-cast module, due to its size and weight, becomes a cost factor in the construction which dictates the desirability of having a number of production sites throughout the country to reduce transportation costs and time, as opposed to one or two centrally located production facilities.
Secondly, production costs for creating and maintaining conventional steel or other forming systems are relatively high, which in effect limits the number of operators which can enter the module production business due to necessary start-up costs. For example, it is estimated that the cost of conventionally fabricated steel forms for a rectangular hollow unit of the type described in this invention, having an approximately ten foot interior opening would be at least $50,000.00; in order to obtain a volume production, a given producer would necessarily have to have a number of such steel forms and the required capital to build or purchase the same.
Thirdly, particularly since the modern trend is to high rise units in which a large number of modules are vertically stacked, dimensional variation from one module to another causes difficulty and in fact often prevents the use of such units because dimensional variations from one unit to another are multiplied as the units become vertically stacked.
Any practical high production application of the pre-cast module concept would require the use of many separate and independent form assemblies and often times independent casting sites and form suppliers as well. With the more conventional form systems used in the industry to date considerable variation in various critical dimensions of concrete module units are experienced. These variations are often caused by dimensional variations between one form assembly and another, and by differences between dimensions enclosed by one particular form assembly set up for one casting operation and the same form assembly set up for another casting operation. Dimensional differences can also be caused by temperature changes, warping or bending of structural members, worn parts and a multitude of other factors. In order to satisfactorily construct a multiple story building of such pre-cast units, it is generally necessary that any dimensional variation from unit to unit, in the type of ten foot units herein described, be limited to very close tolerances. That is, the units must be straight, square, stackable and dimensionally accurate, within, for example, plus or minus 1/8 inch in a ten foot length.
These close dimensional tolerances are especially necessary in large construction projects where modules cast by more than one source or from several separate forms at any one source are necessarily used. As previously stated, it is desirable to have one or more sources for the modules fairly close geographically to the site of the project. Unless the producers of the modules for that project are casting the many individual units from dimensionally identical forms or molds, great difficulty will be encountered by the contractor in assembling the modules from multiple forms or sources due to dimensional variations. The economically impractical alternative to the instant molds, due to the capital requirements, is the conventional system of multiple finished steel molds which increases the contractor's cost and thus the cost to the ultimate project sponsor, public or private.